Design and Analysis of Algorithms 214.

Introduction to the C Programming

Language.

U.U.Samantha Rajapaksha
BSc. Eng. (Moratuwa), MSc in IT
Senior Lecturer
Sri Lanka Institute of Information Technology
New Kandy Road,
Malabe, Sri Lanka
 Tel:0112-301904
email: samantha.r@sliit.lk
Web: www.sliit.lk

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C History
 The C language was first developed in 1972 by Dennis Ritchie
at AT&T Bell Labs. Ritchie called his newly developed language
C simply because there was a B programming language
already. (As a matter of fact, the B language led to the
development of C.)
 C is a high-level programming language. In fact, C is one of the
most popular general-purpose programming languages.
 The first version of Unix was written in the low-level PDP-7
assembler language.
 Ken Thompson developed a compiler for a new high-level
language called B, based on the earlier BCPL language
developed by Martin Richard.

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C History (Contd.)
 BCPL is a simple typeless language .
 BCPL (Basic Combined Programming Language) is a computer
programming language designed by Martin Richards of the
University of Cambridge in 1966.
 Originally intended for writing compilers for other languages,
BCPL is no longer in common use. However, its influence is still
felt due to its role in inspiring Dennis Ritchie's widely-used C
programming language.
 BCPL was the first curly bracket programming language.
 The language is unusual in having only one data type: a word, a
fixed number of bits,
 B was used for further development of the Unix system, which
made the work much faster and more convenient.

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C History (Contd.)
 When the PDP-11 computer arrived at Bell Labs, Dennis Ritchie
built on B to create a new language called C which inherited
Ken Thompson's taste for concise syntax, and had a powerful
mix of high-level functionality and the detailed features required
to program an operating system. Most of the components of
Unix were eventually rewritten in C.

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The C Language
 Currently, the most commonly-used language for embedded
systems
 “High-level assembly”
 Very portable: compilers exist for virtually every processor
 Easy-to-understand compilation
 Produces efficient code
 Designed for systems programming
 Operating systems

 Utility programs

 Compilers

 Filters

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Standard C.
 Standardized in 1989 by ANSI (American National Standards
Institute) known as ANSI C
 International standard (ISO) in 1990 which was adopted by
ANSI and is known as C89
 As part of the normal evolution process the standard was
updated in 1995 (C95) and 1999 (C99)
 C++ and C
 C++ extends C to include support for Object Oriented

Programming and other features that facilitate large software

development projects.

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Writing C Programs
 A programmer uses a text editor to create or modify files
containing C code.
 Code is also known as source code.
 A file containing source code is called a source file.
 After a C source file has been created, the programmer must
invoke the C compiler before the program can be executed
(run).
 we will use the gcc compiler as it is the compiler available on
the Linux system.
 All grading is down on Linux. If you use any other compiler, run
it first on the system where the grading is being done. There
are differences between the compilers and we only support the
gcc compiler on Linux.

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Invoking the gcc Compiler
At the prompt, type
 gcc -ansi -Wall pgm.c
 where pgm.c is the C program source file.
 -ansi is a compiler option that tells the compiler to adhere to the ANSI C
standard.-Wall is an option to turn on all compiler warnings (best for new
programmers).
 If there are no errors in pgm.c, this command produces an executable file, which
is one that can be executed (run).
 The gcc compiler names the executable file a.out .
 To execute the program, at the prompt, type
a.out
 Although we call this process “compiling a program,” what actually happens is
more complicated.
 Create example file: try.c
 Compile using gcc:
gcc –o try try.c
 Execute program
./try

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Stages of Compilation.
Stage 1: Preprocessing

 Performed by a program called the preprocessor.

 Modifies the source code (in RAM) according to
preprocessor directives (preprocessor commands)
embedded in the source code.
 Strips comments and white space from the code.
 The source code as stored on disk is not modified.

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Stages of Compilation. (contd.)
Stage 2: Compilation

 Performed by a program called the compiler

 Translates the preprocessor-modified source code into object
code (machine code)
 Checks for syntax errors and warnings
 Saves the object code to a disk file, if instructed to do so (we
will not do this).
 If any compiler errors are received, no object code file will
be generated.
 An object code file will be generated if only warnings, not
errors, are received.

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Stages of Compilation. (contd.)

Stage 3: Linking

 Combines the program object code with other object code to

produce the executable file.
 The other object code can come from the Run-Time Library, other
libraries, or object files that you have created.
 Saves the executable code to a disk file. On the Linux system, that
file is called a.out.
 If any linker errors are received, no executable file will be
generated.

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Program Development Using gcc.
Editor
Source File pgm.c

Preprocessor

Modified Source Code in RAM

Compiler
Program Object Code File pgm.o
Other Object Code Files (if any)

Linker

Executable File a.out

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A Simple C Program
/*
*************************************************
*** Program: hello_world ***
*** Author: Samantha(samantha.r@sliit.lk) ***
*** Course: DAA 214 Semester two 2012 ***
*** Lab: Lab 01. Fridays 10:30am ***
*** Description: Prints the sentence ***
*** "Hello, world!" to standard output. ***
*************************************************
*/
 #include <stdio.h>
 int main ()
 {
 printf ( “Hello, World!\n” ) ;
 return 0 ;
 }

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Program Header Comment and
Preprocessor Directives
 A comment is descriptive text used to help a reader of the
program understand its content.
 All comments must begin with the characters /* and end with
the characters */
 These are called comment delimiters
 The program header comment always comes first.

 Lines that begin with a # in column 1 are called preprocessor

directives (commands).
 Example: the #include <stdio.h> directive causes the
preprocessor to include a copy of the standard input/output
header file stdio.h at this point in the code.
 This header file was included because it contains information
about the printf ( ) function that is used in this program.

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C Standard Library
 Every implementation of C comes with a standard
library of predefined functions.
 Note that, in programming, a library is a collection
of functions.
 The functions that are common to all versions of C
are known as the C Standard Library.

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 C Standard Library Function
Examples
Function Math
Name Name Value Example
abs(x) absolute value |x| abs(-1) returns 1
sqrt(x) square root x0.5 sqrt(2.0) returns 1.414…
exp(x) exponential ex exp(1.0) returns 2.718…
log(x) natural logarithm ln x log(2.718…) returns 1.0
log10(x) common logarithm log x log10(100.0) returns 2.0
sin(x) sine sin x sin(3.14…) returns 0.0
cos(x) cosine cos x cos(3.14…) returns -1.0
tan(x) tangent tan x tan(3.14…) returns 0.0
ceil(x) ceiling ┌ x┐ ceil(2.5) returns 3.0
floor(x) floor └ x┘ floor(2.5) returns 2.0

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stdio.h and int main ()
 When we write our programs, there are libraries of functions to help
us so that we do not have to write the same code over and over.
 Some of the functions are very complex and long. Not having to
write them ourselves make it easier and faster to write programs.
 Using the functions will also make it easier to learn to program.
 Every program must have a function called main. This is where
program execution begins.
 main() is placed in the source code file as the first function for
readability. There must be a function with this name or gcc can not
successful compile and link your program.
 The reserved word “int” indicates that main() returns an integer
value.
 The parentheses following the reserved word “main” indicate that it
is a function.

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The Function Body
 A left brace (curly bracket) -- { -- begins the body of every
function. A corresponding right brace -- } -- ends the function
body.

 printf (“Hello, World!\n”) ;

 This line is a C statement.

 It is a call to the function printf ( ) with a single argument
(parameter), namely the string “Hello, World!\n”.
 Even though a string may contain many characters, the string
itself should be thought of as a single quantity.
 Notice that this line ends with a semicolon. All statements in C
end with a semicolon.

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return 0 ;
 Because function main() returns an integer value,
there must be a statement that indicates what this
value is.
 The statement
return 0 ;

indicates that main() returns a value of zero to

the operating system.
 A value of 0 indicates that the program successfully
terminated execution.
 Do not worry about this concept now. Just remember
to use the statement.

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Variables and Arithmetic
Expressions.
 Variables in C have the same meaning as variables in algebra.
That is, they represent some unknown, or variable, value.
x=a+b
 Variables in C may be given representations containing multiple
characters. But there are rules for these representations.
 Variable names (identifiers) in C
 May only consist of letters, digits, and underscores

 May be as long as you like, but only the first 31 characters

are significant
 May not begin with a digit

 May not be a C reserved word (keyword)

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Reserved Words (Keywords) in C
auto break int long
case char register return
const continue short signed
Default do sizeof static
double else struct switch
enum extern typedef union
float for unsigned void
goto if volatile while

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Naming Conventions.
 C programmers generally agree on the following conventions for
naming variables.
 Begin variable names with lowercase letters

 Use meaningful identifiers

 Separate “words” within identifiers with underscores or mixed

upper and lower case.

 Be consistent!

 C is case sensitive.
 Use all uppercase for symbolic constants (used in #define
preprocessor directives).
 Note: symbolic constants are not variables, but make the program
easier to read.
 Examples:
 #define PI 3.14159
 #define AGE 52

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What Does a Variable Have?
Every variable has:
 a name (e.g., number of students),
 an address (i.e., a location in memory, such as 123456),
 a data type (e.g., int, float, char), and
 a value (which may be undefined, also known as garbage). The
value is also known as the contents of the variable — that is, the
value is the contents of the variable’s memory location.

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Compile Time and Runtime
 Events that occur while a program is being compiled
are said to happen at compile time.
 Events that occur while a program is running are said
to happen at runtime.
For example, the address of a variable is chosen at
compile time, while its value often is determined at
runtime.

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Declaring Variables
 Before using a variable, you must give the compiler some information about
the variable; i.e., you must declare it.
 The declaration statement includes the data type of the variable.
 Examples of variable declarations:
 int age ;
 float area ;
 When we declare a variable
 Space is set aside in memory to hold a value of the specified data type
 That space is associated with the variable name
 That space is associated with a unique address
 Unless we specify otherwise, the space has no known value.
 Visualization of the declaration age
int age ;
Garbage

FF02 int
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Notes About Variables.
 You must not use a variable until you somehow give it a value.
 You can not assume that the variable will have a value before
you give it one.
 Some compilers do, others do not! This is the source of

many errors that are difficult to find.

 Assume your compiler does not give it an initial value!

 Variables may be given initial values, or initialized, when

declared.

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Displaying Variables.
 Variables hold values that we occasionally want to show the person using the
program.
 We have a function called printf( ) that will allow us to do that.
 The function printf needs two pieces of information to display things.
 How to display it
 What to display
 printf( “%f\n”, diameter );
 The name of the function is “printf”.

 The %f is known as a placeholder: it holds the place of the value of the variable
that we actually want to output.

 Inside the parentheses are:

 print specification, where we are going to display:
 a floating point value (“%f”)
 We want to have the next thing started on a new line (“\n”).
 We want to display the contents of the variable diameter.

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Code Format
%c character
%d signed integers
%i signed integers
%e scientific notation, with a lowercase "e"
%E scientific notation, with a uppercase "E"
%f floating point
%g use %e or %f, whichever is shorter
%G use %E or %f, whichever is shorter
%o octal
%s a string of characters
%u unsigned integer
%x unsigned hexadecimal, with lowercase letters
%X unsigned hexadecimal, with uppercase letters
%p a pointer
the argument shall be a pointer to an integer into which is
%n
placed the number of characters written so far
%% a '%' sign
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scanf (“%f”, &diameter);
 The scanf( ) function also needs two items:
 The input specification “%f”. (Never put a “\n” into the input

specification.)
 The address of where to store the information. (We can

input more than one item at a time if we wish, as long as we

specify it correctly.)
 Notice the “&” in front of the variable name. It says to use the
address of the variable to hold the information that the user
enters.
 Comments should explain why you are doing something, not
what you are doing it.

DAA 214 Lab 01 Sri Lanka Institute of Information Technology.

Reading Multiple Variables with a
Single scanf
C allows inputting multiple variables per scanf statement. At
runtime, when the user types in the input values, they can
separate the individual input values
 by blank spaces, and/or
 by tabs, and/or
 by carriage returns (newlines).
 Blank spaces, tabs and carriage returns, as a group, are known as
white space.

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Exercise 01.
 Write a program to read two integers from the stdin and output
the sum of two integers them to stdout.

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Arithmetic Operators in C
 Name Operator Example

 Addition + num1 + num2

 Subtraction - initial - spent
 Multiplication * fathoms * 6
 Division / sum / count
 Modulus % m%n
 If both operands of a division expression are integers, you will
get an integer answer. The fractional portion is thrown away.
 Examples : 17 / 5 = 3
 4 / 3 = 1
 35 / 9 = 3

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Division By Zero.
 Division by zero is mathematically undefined.
 If you allow division by zero in a program, it will cause a fatal
error. Your program will terminate execution and give an error
message.
 Non-fatal errors do not cause program termination, just
produce incorrect results.

DAA 214 Lab 01 Sri Lanka Institute of Information Technology.

Arithmetic Operators
Rules of Operator Precedence
 Operator (s) Precedence & Associativity

() Evaluated first. If nested
(embedded), innermost first. If
on same level, left to right.
 * / % Evaluated second. If there are
several, evaluated left to right.
 + - Evaluated third. If there are
several, evaluated left to right.
 = Evaluated last, right to left.

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What is a Constant?
 In mathematics, a constant is a value that cannot change.
 In programming, a constant is like a variable, except that its value cannot
change.

There are two categories of constants:

 literal constants, whose values are expressed literally;
 named constants, which have names.
A literal constant is a constant whose value is specified literally:
 int literal constants

(e.g., 2, 1, 7,98, -1)

 float literal constants

(e.g., 2.4, 0.6, -14.7)

 char literal constants

(e.g., ’B’, ’4’, ’?’)

DAA 214 Lab 01 Sri Lanka Institute of Information Technology.

Named Constants
 A named constant is a constant that has a name.
 A named constant is exactly like a variable, except that its value
is set at compile time and CANNOT change at runtime.
 A named constant is exactly like a literal constant, except that it
HAS A NAME.
 In a named constant declaration, we indicate that it’s a constant
via the const attribute, and we MUST initialize it:
const float pi = 3.14;
 When you embed numeric literal constants in the body of your
program, you make it much harder to maintain and upgrade
your program.

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Repetition and Looping
 Repetition means performing the same set of statements over
and over.
 The most common way to perform repetition is via looping.
 A loop is a sequence of statements to be executed, in order, over
and over, as long as some condition continues to be true.
 while Loop
 C has a loop construct known as a while loop:
 while (condition) {

statement1;

statement2;

...

}
 The condition of a while loop is a Boolean expression completely
enclosed in parentheses – just like in an if block.
 The sequence of statements between the while statement’s block
open and block close is known as the loop body.

DAA 214 Lab 01 Sri Lanka Institute of Information Technology.

while Loop Behavior
while (condition) {
statement1;
statement2;
...
}
A while loop has to the following behavior:
1. The condition is evaluated, resulting in a value of either true (1) or
false (0).
2. If the condition evaluates to false (0), then the statements inside the
loop body are skipped, and control is passed to the statement that is
immediately after the while loop’s block close.
3. If the condition evaluates to true (1), then:
a. the statements inside the loop body are executed in sequence.
b. When the while loop’s block close is encountered, the program
jumps back up to the associated while statement and starts over
with Step 1.

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while Loop Flowchart
statement_before;
while (condition) {
statement_inside1;
statement_inside2;
...
}
statement_after;

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for Loop
for (counter = initial_value;
counter <= final value; counter++) {
statement1;
statement2;
...
} /* for counter */
statement_after;

DAA 214 Lab 01 Sri Lanka Institute of Information Technology.

Variables and Arithmetic
Expressions.
 Exercise 01: Write the program to print the following table of
Fahrenheit temperatures and their centigrade or Celsius equivalents.
[ Use the formula c = 5/9( f – 32).]
0 -17
20 -6
40 4
60 15
. .
 Exercise 02: Modify the program to print the following table.
0 -17.8
20 -6.7
40 4.4
...

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